Gas-generating pyrotechnical compositions and pyrotechnical compounds for automotive safety

ABSTRACT

The present invention provides a pyrotechnic gas generating composition and pyrotechnic compounds able to be obtained from said composition. Said composition comprises basic copper nitrate, guanidine nitrate, a specific additional reducing charge, and/or a specific additional oxidizing charge as well as a specific binder.

The present invention relates to pyrotechnic gas generation, inparticular to inflate airbags used in systems for protecting theoccupants of a motor vehicle. More particularly, the invention relatesto pyrotechnic compositions generating, at temperatures which areacceptable to automotive safety, clean gases termed “cold” gases whichare rich in nitrogen and are non-toxic, and it also relates topyrotechnic compounds able to be obtained from such compositions.

For various pyrotechnic requirements, in particular to ensure thecorrect inflation of airbags, pyrotechnic gas generators must, overextremely short periods of the order of thirty milliseconds, provideclean gases, ie. which are free of solid particles which couldconstitute hot spots and which might damage the airbag wall, and whichare non-toxic, i.e. containing small quantities of oxides of nitrogen,oxides of carbon and chlorinated products.

U.S. Pat. No. 5,608,183 discloses pyrotechnic gas-generatingcompositions comprising a reducing charge constituted by guanidinenitrate (GN) and an oxidizing charge constituted by basic copper nitrate(BCN) in predetermined proportions. Such compositions are veryadvantageous since they burn at low temperatures, less than 2000 K,allowing them to be used in gas generators for airbags, and since theycan produce high gas yields during combustion thereof.

However, as described, for example, in U.S. Pat. No. 6,143,102, suchcompositions have low combustion rates, in particular due to thepresence of guanidine nitrate in the composition (see also column 3 ofU.S. Pat. No. 6,550,808). Further, it has been noted that suchcompositions are difficult to ignite and they have very low combustiontemperatures, which greatly reduces their inflation power (“inflationpower” is defined as the product of the gas yield on combustionmultiplied by its combustion temperature). When used in the automotivesafety field to inflate an airbag, the combustion temperature of thecomposition must remain low, if possible below 2200 K. In fact, attemperatures of less than 2200 K, it is still possible to use airbagswith an inner wall which is uncoated. From a cost viewpoint, this mayprove to be decisive in a market which is as competitive as theautomobile market. However, too low a combustion temperature penalizesthe inflation power of the composition excessively.

European patent applications EP-A-1 279 655 and EP-A-1 130 008 describemethods of producing gas-generating compounds. Long lists are providedof substances which may be suitable as constituents of those compounds,as the reducing charge, as the oxidizing charge, and as additives. Theproduction of small-diameter mono-perforated monolithic compounds isdescribed in general manner.

Thus, the invention aims to provide an extrudable pyrotechniccomposition (which, in general, is suitable for preparing mono- ormulti-perforated monolithic compounds) which is capable of generatingclean, non-toxic gases at low temperatures of less than 2300 K, allowingit to be used to inflate an uncoated airbag, which ignites readily, andwhich has a satisfactory inflation power.

This aim is achieved by a pyrotechnic gas-generating compositioncomprising an oxidizing charge constituted by basic copper nitrate(BCN), a reducing charge constituted by guanidine nitrate (GN) and abinder, the composition being characterized in that it also comprises:

an additional reducing charge selected from the group formed by hexogene(RDX), octogene (HMX), penthrite (PETN), triaminoguanidine nitrate(TAGN), nitroguanidine, 3-nitro-1,2,4-triazol-5-one (ONTA) and mono- andbi-tetrazoles; and/or, advantageously and

an additional oxidizing charge which forms a solid solution obtained bysubstitution with guanidine nitrate (GN); and

-   -   in that the binder, which is hydrosoluble, is based on a mixture        of at least one carboxymethylcellulose with a high molecular        mass and at least one carboxymethylcellulose with a low        molecular mass, in a mass ratio in the range 95/5 to 60/40.

According to the invention, basic copper nitrate (hereinafter BCN) withformula Cu(NO₃)₂, 3Cu(OH)₂, is selected as the oxidizer since it has theadvantage of being perfectly stable and, combined with a reducing agent,of burning by forming copper residues that are easy to filter. Further,BCN is insoluble in water, which is advantageous when the compositionuses a hydrosoluble binder which allows it to be fabricated byextrusion. BCN also has a satisfactory gas yield which is higher thanthat of compounds such as copper oxide, and a relatively high oxygenbalance (OB) of +30%.

In an advantageous implementation of the invention, the basic coppernitrate (BCN) is present in a mass fraction in the range 40% to 60%(highly advantageously in the range 50% to 60%) of the total compositionmass.

According to the invention, the selected reducing charge is guanidinenitrate (GN). Guanidine nitrate (GN) is an organic compound which isrich in nitrogen, stable, and inexpensive. Guanidine nitrate (GN) hasgood ageing properties, as measured by the 400 hour test at 107° C.Further, the presence of guanidine nitrate (GN) in the compositionimproves the gas yield of the composition. Guanidine nitrate (GN) has anegative enthalpy of formation, also having the effect of reducing thecombustion temperature of the composition.

In an advantageous embodiment of the invention, the guanidine nitrate(GN) is present in a mass fraction in the range 20% to 55% (highlyadvantageously in the range 20% to 40%) of the total composition mass.

The presence of the additional reducing charge in the composition of theinvention notably allows to improve the gas yield and to facilitateignition of said composition, thus rendering it more reliable, andavoiding the need for an ignition charge. Ignition charges areexpensive; thus, using a composition of the invention can reduce thecost of the gas generator.

In a preferred embodiment, the selected additional reducing charge ishexogene (RDX) or octogene (HMX).

Said additional reducing charge, when present, is generally present in amass fraction of less than 15% with respect to the total compositionmass.

The presence of a specific additional oxidizing charge in thecomposition of the invention can in particular:

improve the gas yield;

facilitate ignition of said composition; and also

improve the combustion rate of said composition.

It also facilitates implementation of the method of producing saidcomposition, and of producing compounds from said composition.

The additional oxidizing charge able to be used is thus not anyoxidizing charge. It is selected for forming a solid solution obtainedby substitution with the guanidine nitrate (GN).

This type of solid solution resulting from substitution (eutectic) isknown per se to the skilled person. The chemical substances in question(in the present case guanidine nitrate and the second oxidizer) musthave:

a similar molecular size;

the same type of crystal lattice; and

the same valency (or oxidation number).

Entirely surprisingly, the inventors have discovered the greatimportance of this type of solution in the context of the invention. Theimpact on the combustion rate is considerable.

Amongst the numerous known oxidizers already used in the pyrotechnicsfield, only those which form a solid solution obtained by substitutionwith guanidine nitrate are suitable for use in compositions of theinvention. In particular, ammonium perchlorate, potassium perchlorate,ammonium nitrate, sodium nitrate, and potassium nitrate are suitable.Ammonium perchlorate and potassium perchlorate are particularlysuitable. The use of ammonium perchlorate is particularly recommended.These particular oxidizers are dealt with below.

In general, said additional oxidizing charge, when present, is presentin a mass fraction of less than 15%, advantageously less than 10%, ofthe total composition mass.

Ammonium perchlorate is a very strong oxidizer and has a very good gasyield. Its highly oxidizing nature allows the proportion of reducingcharge, and thus of guanidine nitrate (GN) in the composition, to beincreased, GN also having a very good gas yield. Like the additionalreducing charge, the presence of ammonium perchlorate in the compositioncan facilitate ignition of the composition. In the automotive safetyfield, almost all of the pyrotechnic composition must burn at a pressureof the order of 20 MPa [megapascals] in 30 ms [milliseconds] to 40 msfor use in a passenger or driver airbag, or in 20 ms in a side airbag.These combustion times must be adhered to in a composition intended forautomotive safety. When present, ammonium perchlorate acts as a“booster” in the composition.

Further, when the pyrotechnic product obtained burns in parallel layers,the combustion rate V_(c) satisfies the following law:V _(c) =a*p ^(n)in which a is a constant and n the pressure exponent. Combustion of apyrotechnic compound is above all a chemical reaction and is thustemperature-dependent. Using ammonium perchlorate can reduce thepressure exponent, which renders the composition less dependent onpressure and thus less temperature-dependent. In the present invention,the pressure exponent is particularly low, considerably less than 0.7,which means that the pyrotechnic compound can be caused to function attemperatures in the range −35° C. to 85° C. The composition of theinvention may thus be used in an automotive vehicle.

Even though ammonium perchlorate or potassium perchlorate is a strongoxidizer and has advantageous combustion properties, its use in acomposition leads to the production of high combustion temperatures andto the generation of combustion gases which can contain a fairly highlevel of hydrogen chloride. To avoid these problems, it is highlyrecommended that these oxidizers be used in small amounts, generallyless than 15%, preferably less than 10% (of the composition mass). Anindication of these maximum values of 15% or 10% has already been givenin a general manner with reference to the use of any suitable additionaloxidizer.

The composition of the invention is also capable of including atransition metal oxide to catalyze decomposition of the additionaloxidizer. Said transition metal oxide may in particular be an oxide ofiron, copper, or manganese, generally present in an amount of less than5% by weight.

It is stated above that compositions of the invention include anadditional reducing charge as described above and/or an additionaloxidizing charge as described above. Advantageously, they include saidadditional reducing charge and said additional oxidizing charge.

Thus, they are advantageously of the type including:

basic copper nitrate (BCN) and an additional oxidizing charge; and

guanidine nitrate (GN) and an additional reducing charge.

Said specific oxidizer(s) and reducer(s) are, in a characteristicmanner, included in a specific hydrosoluble binder. Said specifichydrosoluble binder is advantageous:

in that it is hydrosoluble. (Some patent applications, such as Frenchpatent application FR-A-2 772 370, describe the use of a cross-linkedreducing binder based on silicone resin or based on epoxy resin. Thebinder is present in that composition in an amount which isadvantageously in the range 6% to 10% of the total composition weight.To be suitable for production by extrusion with a twin-screw extruder, acomposition must have a binder content of the order of 4% or 5%, forexample of the total composition mass and necessitates the use of asolvent for the binder. The binders used in that prior art compositionrequire the use of organic or halogenated solvents. However, the use ofhalogenated solvents such as trichloroethylene is regulated, whichgreatly complicates the manufacture of such a composition and increasesits manufacturing cost. Similarly, the use of organic solvents such asketone type solvents (acetone, methylethylketone, etc, for example)requires the use of complex solutions to control the emissions ofvolatile organic compounds (VOC). The hydrosoluble binder in thecompositions of the invention avoids having to use such organic orhalogenated solvents during granulation or extrusion steps involvedpyrotechnic compounds); and

in that it is based on a mixture of at least one carboxymethylcellulosewith a high molecular mass and at least one carboxymethylcellulose witha low molecular mass in a mass ratio in the range 95/5 to 60/40; i.e. inthat it includes long fibers and short fibers, in the ratio indicatedabove.

The notions of high and low molecular masses are defined below. Clearly,they refer to mean molecular masses. The term “high molecular mass” asused in the present application and claims means a mean molecular massof more than 250,000 g/mol [grams/mol], advantageously 700,000 g/mol ormore. The term “low molecular mass” as used in the present applicationand claims means a mean molecular mass of less than 100,000 g/mol,generally much less than 90,000 g/mol. These definitions will come as nosurprise to the skilled person.

Carboxymethylcellulose performs well since it has:

good ageing behavior, in particular as measured in an ageing testcarried out for 400 hours at 107° C.; and

an advantageous oxygen balance. Compositions used in the automotivesafety field must have an equilibrated oxygen balance (OB). Bydefinition, a composition is termed “equilibrated” in oxygen when thecomposition comprises sufficient oxygen so that after reaction, thevarious compounds of the composition are in the form of CO₂, H₂O and N₂.The oxygen balance of the binder must be as low as possible.Carboxymethylcellulose has a much higher oxygen balance than the oxygenbalance of elastomeric type binders.

Carboxymethylcellulose, a mixture of short and long fibers as definedabove, performs particularly well in operations of granulation,compression, and extrusion, and is entirely suitable for processing thecomposition into the form of pellets, disks, or monolithic blocks, whichare mono- or even multi-perforated. The mixture of short and long fiberscan produce:

in solution, in a minimum of water, a continuously pumpable andmeterable gel, accepting a large amount of solid charge; and

an extruded product which does not deform under its own weight.

The specific hydrosoluble binder as defined above is generally presentin compositions of the invention in a mass fraction in the range 2% to15% of the total composition mass. Beyond 15%, its influence may bedeleterious to the oxygen balance of the composition.

The specificity of the binder for the compositions of the inventionmeans that compounds can be obtained by continuous extrusion using acontinuous twin-screw type extruder, which compounds have aconfiguration described in the profession as a monolithic block (orgrain) with one or multiple perforations, with an external diameter of afew millimeters to about twenty millimeters. The number of perforationsmay vary from 1 to 19 for perforations 0.5 mm [millimeters] to 1.5 mm indiameter.

The composition of the invention may also include additives, inparticular additives acting as catalysts for combustion or agents fortrapping solid particles produced during combustion. Said agents cantrap solid particles produced during combustion to create residues whichare of a sufficient size to allow them to be filtered. Additives whichare well known in the field of compositions for automotive safety, forexample alumina or silica, may be added to the composition of theinvention.

In a second aspect, the present invention provides compounds which areable to be obtained from the pyrotechnic compositions described above.

In a first variation, pyrotechnic compounds having a composition asdescribed above are manufactured and shaped by a pelletization or diskcompression process.

In a second variation, pyrotechnic compounds having a composition asdescribed above are manufactured and shaped by an extrusion process.

In the invention, pyrotechnic compounds having a composition of theinvention can be mass produced by disk compression or pelletization.

Before the pelletization operation, a step of preparing a powder must becarried out. This step is not simply a matter of mixing the various dryconstituents. To carry out the pelletization operation, it is necessaryto obtain a powder that flows properly. This preparation step is agranulation operation consisting in starting from the various materialsconstituting the composition of the invention in powder form to makegrains of larger grain size, of the order of a few hundred microns. Oncethis powder has been obtained, the pelletization operation can becarried out.

These pelletization or compression processes per se are known to theskilled person.

In the invention, pyrotechnic compounds having a composition of theinvention may also be obtained by extrusion. Extrusion is renderedpossible, is indeed optimized by the presence of carboxymethylcellulosewith added water. Assuming that ammonium perchlorate or potassiumperchlorate is used, a small quantity of water should be incorporated toprevent said perchlorate from dissolving. The method of producingpyrotechnic compounds by extrusion advantageously comprises a step forcontinuously supplying a mixing and extrusion apparatus such as atwin-screw extruder with, on the one hand, the oxidizing charge(s) (BCNand the optional additional oxidizing charge) and with, on the otherhand, the reducing charge(s) (GN and the optional additional reducingcharge) pre-mixed with the binder used (the specific binder used). Aftermixing, using the extruder, a section rod is extruded which isoven-hardened, and then cut to the desired length, for example into theform of granulates. Cutting may also be carried out, in certain cases inwhich dimensional requirements are not so severe, before oven hardening.The monolithic compounds obtained may be mono- or multi-perforated. Thespecificity of the binder (mixture of short and long fibers) allows thecompounds to be obtained in the multi-perforated form.

Table 1 below shows a few examples of the formulations for thecompositions of the invention. The percentages indicated are percentagesby weight. TABLE 1 C* Ex** CMC-Na BCN GN RDX HMX ONTA O.M. NH₄ClO₄ NaNO₃KClO₄ Ref 3.6 52.4 41 3 1 8 59.6 22.4 10 2 3.6 43.1 42.7 3 7.6 3 2 53.439.6 5 4 2 52.4 30.6 15 5 4 50.8 30.2 10 5 6 4 54.1 38.9 3 7 8 60.3 21.710 8 4 52 34 5 5 9 4 58.9 27.1 10 10 2 46.2 36.8 12.3 2.7 11 4 42 43.92.7 4.3 3.1C*: constituentsEx**: examples

The following abbreviations were used in this table:

-   CMC-Na=sodium carboxymethylcellulose (quantity expressed as a    percentage). The binder used was a mixture of CMC-Na with a high    mean molecular mass: Mw≈700,000 g/mol (BLANOSE® from AQUALON    HERCULES—grade 7H—) and CMC-Na with a low mean molecular mass:    Mw<<90000 g/mol (BLANOSE® from AQUALON HERCULES—grade 12UL—). The    mixtures in question were generally 85/15 mixtures (mass ratio: high    molecular mass CMC-Na/low molecular mass CMC-Na).-   BCN=basic copper nitrate (quantity expressed as a percentage)-   GN=guanidine nitrate (quantity expressed as a percentage)-   RDX=hexogene (quantity expressed as a percentage)-   HMX=octogene (quantity expressed as a percentage)-   ONTA=3-nitro-1,2,4-triazol-5-one (quantity expressed as a    percentage)-   O.M.=metallic oxide (quantity expressed as a percentage) such as    SiO₂ or Al₂O₃, used as a ballistic catalyst and/or particle trapping    agent.

For the compositions of Table 1 above, the theoretical results shown inTable 2 below are obtained. TABLE 2 O.B. Yield Tc Ex (%) (mol/100 g) (K)Ref −2.2 2.65 1819 1 −2 2.43 2031 2 −2.2 2.85 2001 3 −1 2.70 1962 4 −12.65 2205 5 −0.5 2.65 2156 6 −0.5 2.65 1873 7 −2 2.38 1920 8 +0.5 2.521959 9 +0.5 2.44 1987 10 −3.5 2.80 2178 11 −3 2.84 1959O.B. = oxygen balance (as a percentage)Yield = gas yield (as moles per kg of burned composition)Tc = combustion temperature (K).

Table 2 above shows that by adjusting the equivalent oxygen balance(OB), the proposed novel formulations allow an increase in the gas yieldand/or the combustion temperature, which results in a larger gas volumeobtained for the same mass of propergol. Similarly, as these examplesshow, it is also possible to adjust the OB of said compositions to adaptit to the requirements of each application. The good performance of saidcompositions is confirmed by the results in the manometric chamber(Table 4 below).

Table 3 below shows the results obtained for firing 30 g [grams] of acomposition in accordance with the various examples of Table 1 into a 60L [liter] tank. TABLE 3 Ex Tall18 (ms) Tt 0.9 (ms) P_(max tank) (MPa)Ref 3.1 46 0.18 2 2.5 38 0.21 4 2.4 40 0.23Tall18 = time to reach a pressure of 18 MPa in the generator chamberused for the tests;Tt0.9 = time between firing and the time at which the tank pressure is90% of the maximum pressure in the tank.

The results shown in Table 4 below were obtained when firing in themanometric tank: TABLE 4 Ex V_(c) (20 MPa) mm/s Pressure exponent Ref14.2 0.5 (11-24 MPa) 2 22 0.7 (16-37 MPa) 10 18 0.3 (15-35 MPa) 11 22 1(15-35 MPa)V_(c) = combustion rate.

In the above reference example, the composition was ignited with 140 mg[milligrams] of TiPP powder (powder based on titanium and potassiumperchlorate) and with 450 mg of ignition charge. In contrast, for thecompositions of Examples 2, 10 and 11, ignition was carried out withonly 140 mg of TiPP. This shows that the presence of ammoniumperchlorate or RDX in the compositions can greatly improve ignition andmeans that ignition charge can be dispensed with.

1. A pyrotechnic gas-generating composition comprising an oxidizingcharge constituted by basic copper nitrate (BCN), a reducing chargeconstituted by guanidine nitrate (GN) and a binder, the compositionbeing characterized in that it also comprises: an additional reducingcharge selected from the group formed by hexogene (RDX), octogene (HMX),penthrite (PETN), triaminoguanidine nitrate (TAGN), nitroguanidine,3-nitro-1,2,4-triazol-5-one (ONTA) and mono- and bi-tetrazoles; and/or,advantageously and an additional oxidizing charge which forms a solidsolution obtained by substitution with guanidine nitrate (GN); and inthat the binder, which is hydrosoluble, is based on a mixture of atleast one carboxymethylcellulose with a high molecular mass and at leastone carboxymethylcellulose with a low molecular mass, in a mass ratio inthe range 95/5 to 60/40.
 2. The composition according to claim 1,characterized in that the basic copper nitrate (BCN) is present in amass fraction in the range 50% to 60% of the total composition mass. 3.The composition according to claim 1, characterized in that theguanidine nitrate (GN) is present in a mass fraction in the range 20% to40% of the total composition mass.
 4. The composition according to claim1, characterized in that the additional reducing charge is hexogene(RDX) or octogene (HMX).
 5. The composition according to claim 1,characterized in that additional reducing charge is present in a massfraction of less than 15% with respect to the total composition mass. 6.The composition according to claim 1, characterized in that theadditional oxidizing charge which is present is selected from the groupformed by ammonium perchlorate, potassium perchlorate, ammonium nitrate,sodium nitrate and potassium nitrate.
 7. The composition according toclaim 1, characterized in that the additional oxidizing charge which ispresent is selected from the group formed by ammonium perchlorate andpotassium perchlorate; and in that said additional oxidizing chargeadvantageously consists of ammonium perchlorate.
 8. The compositionaccording to claim 1, characterized in that the additional oxidizingcharge is present in a mass fraction of less than 15% of the totalcomposition mass.
 9. The composition according to claim 1, characterizedin that the binder is present in a mass fraction in the range 2% to 15%of the total composition mass.
 10. Pyrotechnic compounds able to beobtained from a composition according to claim
 1. 11. The pyrotechniccompounds according to claim 10, manufactured and formed by apelletization or disk compression process.
 12. The pyrotechnic compoundsaccording to claim 10, manufactured and formed by an extrusion process.13. The pyrotechnic compounds according to claim 10, of the monolithic,mono- or multi-perforated type.
 14. The pyrotechnic compounds accordingto claim 12, of the mono- or multi-perforated type.